Plasticity in leaf-level water relations of tropical rainforest trees in response to experimental drought

Abstract

The tropics are predicted to become warmer and drier, and understanding the sensitivity of tree species to drought is important for characterizing the risk to forests of climate change. This study makes use of a long-term drought experiment in the Amazon rainforest to evaluate the role of leaf-level water relations, leaf anatomy and their plasticity in response to drought in six tree genera. The variables (osmotic potential at full turgor, turgor loss point, capacitance, elastic modulus, relative water content and saturated water content) were compared between seasons and between plots (control and through-fall exclusion) enabling a comparison between short- and long-term plasticity in traits. Leaf anatomical traits were correlated with water relation parameters to determine whether water relations differed among tissues. The key findings were: osmotic adjustment occurred in response to the long-term drought treatment; species resistant to drought stress showed less osmotic adjustment than drought-sensitive species; and water relation traits were correlated with tissue properties, especially the thickness of the abaxial epidermis and the spongy mesophyll. These findings demonstrate that cell-level water relation traits can acclimate to long-term water stress, and highlight the limitations of extrapolating the results of short-term studies to temporal scales associated with climate change.

Keywords: Amazon rainforest; experimental drought; leaf anatomy; osmotic adjustment; plasticity; water relations.

Figure 1

Comparison between seasonal and plot effects of pressure volume parameters in 44 tropical rainforest trees from six genera. (a) Comparison of plots. White bars, control plot; gray bars, through‐fall exclusion plot (TFE). (b) Comparison of seasons. White bars, wet season; gray bars, dry season. Bars display the mean ± 1 SE and significance is denoted by asterisks: *, < P = 0.05; **, < P = 0.01; ***, < P = 0.001; P = 0.05 < • < P = 0.10. Annual rain in the drought plot is ≈90 mm per month, in the control plot is ≈180 mm per month, in the wet season (averaged between the TFE and control plot) is ≈210 mm per month and in the dry season is ≈60 mm per month. ${\mathrm{\Psi }}_{\mathrm{\pi }}^{\mathrm{tlp}}$ _, turgor loss point; ${\mathrm{\Psi }}_{\mathrm{\pi }}^{\mathrm{o}}$, osmotic potential at full turgor; SWC, saturated water content; RWC ^tlp _is, relative water content at ${\mathrm{\Psi }}_{\mathrm{\pi }}^{\mathrm{tlp}}$.

Figure 2

Plot (control and through‐fall exclusion (TFE)) and season (wet and dry) effects on hydraulic capacitance in 44 tropical rainforest trees from six genera. Both the seasonal effect (P = 0.007) and the interaction between season and drought treatment (P = 0.014) are significant. Gray bars, dry season; white bars, wet season. Bars display the mean ± 1 SE.

Figure 3

Season and drought sensitivity status effects for osmotic potential at full turgor (${\mathrm{\Psi }}_{\mathrm{\pi }}^{\mathrm{o}}$; P < 0.001), osmotic potential at turgor loss point (${\mathrm{\Psi }}_{\mathrm{\pi }}^{\mathrm{tlp}}$; P < 0.001) and hydraulic capacitance (P = 0.044) in 44 tropical rainforest trees from six genera. Gray bars, drought‐resistant species; white bars, drought‐sensitive species. Bars display the mean ± 1 SE.

Figure 4

Relationships between pressure–volume parameters and tissue thickness in 28 tropical rainforest trees from six genera. The Pearson product–moment correlation coefficient for: (a) is r = −0.44, (b) is r = 0.32, (c) is r = 0.47, and (d) is r = −0.55. ${\mathrm{\Psi }}_{\mathrm{\pi }}^{\mathrm{tlp}}$ is turgor loss point and ${\mathrm{\Psi }}_{\mathrm{\pi }}^{\mathrm{o}}$ is osmotic potential at full turgor.